An optical fiber's refractive-index profile is generally described as a relationship between refractive index and optical-fiber radius. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The outer cladding, functioning as an optical cladding, typically has a refractive index that is substantially constant. This outer cladding is typically made of pure silica but may also contain one or more dopants.
The refractive-index profile may have a “step” profile, a “trapezoidal” profile, a “parabolic” profile (e.g., an “alpha” profile), or a “triangular” profile, which can be graphically depicted as a step, trapezoidal, parabolic, or triangular shape, respectively. These curves are generally representative of the theoretical or design profile of the optical fiber. Constraints associated with optical-fiber fabrication may lead in practice to a profile that is perceptibly different.
An optical fiber conventionally includes an optical core, which has the function of transmitting and optionally amplifying an optical signal. A conventional optical fiber also typically includes an optical cladding, which confines the optical signal in the core. For this purpose, the refractive index of the core nc is typically greater than the refractive index of the cladding ng (i.e., nc>ng). As will be understood by those having ordinary skill in the art, the propagation of an optical signal in a single-mode optical fiber includes a fundamental mode, typically denoted LP01, which is guided in the core, and secondary modes, which are guided over a certain distance in the core and the optical cladding.
Single-mode optical fibers (SMFs) with a step-index profile are often used within optical-fiber transmission systems. Such optical fibers typically possess a chromatic dispersion and a chromatic-dispersion slope that comply with specific telecommunications standards.
Conventionally, so-called “standard” single-mode fibers (SSMFs) are used for land-based transmission systems. To facilitate compatibility between optical systems from different manufacturers, the International Telecommunication Union (ITU) has defined a standard reference ITU-T G.652 with which a standard optical transmission fiber (i.e., a standard single-mode fiber or SSMF) should comply. The ITU-T G.652 recommendations (11/2009) and each of its attributes (i.e., A, B, C, and D) are hereby incorporated by reference.
Among other recommendations for a transmission fiber, the ITU-T G.652 standard recommends (i) a mode field diameter (MFD) with a nominal value (e.g., a nominal mode field diameter) of between 8.6 microns (μm) and 9.5 microns and a tolerance of ±0.6 micron at a wavelength of 1310 nanometers (nm), (ii) a maximum cable cut-off wavelength (λCC) of 1260 nanometers (nm), (iii) a zero-dispersion wavelength (ZDW) of between 1300 nanometers and 1324 nanometers, and (iv) a maximum zero-dispersion slope (ZDS) of 0.092 picoseconds per square nanometer kilometer (ps/(nm2·km)) (i.e., the chromatic-dispersion slope at the zero-chromatic-dispersion wavelength is 0.092 ps/(nm2·km) or less).
The cable cut-off wavelength is conventionally measured as being the wavelength at which the optical signal is no longer single mode after propagating over 22 meters in the optical fiber, as defined by subcommittee 86A of the International Electrotechnical Commission (IEC) in standard IEC 60793-1-44. The IEC 60793-1-44 is hereby incorporated by reference in its entirety.
In most circumstances, the secondary mode that best withstands bending losses is the LP11 mode. The cable cut-off wavelength is thus the wavelength from which the LP11 mode is sufficiently attenuated after propagating for 22 meters in an optical fiber. The method proposed by the ITU-T G.652 standard considers that the optical signal is single mode as long as the attenuation of the LP11 mode is greater than or equal to 19.3 decibels (dB). According to the recommendations of IEC subcommittee 86A in standard IEC 60793-1-44, the cable cut-off wavelength is determined by imparting two loops having a radius of 40 millimeters (mm) in the optical fiber, while arranging the remainder of the optical fiber (i.e., 21.5 meters of optical fiber) on a mandrel having a radius of 140 millimeters.
The International Telecommunication Union (ITU) has defined a standard reference ITU-T G.654 with which a cut-off shifted single-mode optical fiber should comply.
The ITU-T G.654 recommendations (12/2006) and each of its attributes (i.e., A, B, and C) are hereby incorporated by reference. The ITU-T G.654.B standard recommends (i) a cable cut-off wavelength (λcc) of 1530 nanometers or less, (ii) a mode field diameter (MFD) with a nominal value (e.g., a nominal mode field diameter) of between 9.5 microns and 13.0 microns with a tolerance of ±0.7 micron at a wavelength of 1550 nanometers, (iii) chromatic dispersion (D) of less than 22 ps/(nm·km) at a wavelength of 1550 nanometers, (iv) a chromatic-dispersion slope (S) of less than 0.070 ps/(nm2·km) at a wavelength of 1550 nanometers, and (v) macrobending losses of less than 0.5 dB/100 turns around a bend radius of 30 millimeters at a wavelength of 1625 nanometers.
The ITU-T G.654.0 standard recommends (i) a cable cut-off wavelength (λcc) of 1530 nanometers or less, (ii) a mode field diameter (MDF) with a nominal value (e.g., a nominal mode field diameter) of between 9.5 microns and 10.5 microns with a tolerance of ±0.7 micron at a wavelength of 1550 nanometers, (iii) chromatic dispersion of less than 20 ps/(nm·km) at a wavelength of 1550 nanometers, (iv) a chromatic-dispersion slope of less than 0.070 ps/(nm2·km) at a wavelength of 1550 nanometers, and (v) macrobending losses of less than 0.5 dB/100 turns around a bend radius of 30 millimeters at a wavelength of 1625 nanometers.
Optical fibers can have pure-silica cores. The absence of dopant in the core of a Pure-Silica-Core Fiber (PSCF) makes it possible to limit optical losses and notably the attenuation at a wavelength of 1550 nanometers. A PSCF therefore typically has a cladding formed of silica doped with fluorine to reduce its refractive index and ensure that the optical signal is confined within the core.
Conventionally, an optical fiber is drawn from an optical-fiber preform in a fiber-drawing tower. The operation of drawing down an optical fiber to scale includes placing the optical-fiber preform vertically in a tower and drawing a strand of optical fiber from one end of the preform. For this purpose, a high temperature is applied locally to one end of the optical-fiber preform until the silica is softened, and then the speed of fiber-drawing and the temperature are continuously regulated to control the diameter of the optical fiber. The optical-fiber preform must present the same ratio of core diameter to cladding diameter as is to be achieved in the optical fiber drawn therefrom.
An optical fiber may be fabricated from an optical-fiber preform that includes a primary preform constituted by a deposition tube of pure or doped silica in which layers of doped and/or pure silica are deposited in succession in order to form an inner cladding and a central core. Primary preforms of this nature are typically fabricated on a deposition bench. The primary preform is then overcladded (e.g., fitted with a sleeve) to increase its diameter and form an optical-fiber preform or final preform that is suitable for use in a fiber-drawing tower. In this context, the term “inner” cladding designates the cladding formed inside the deposition tube (e.g., a substrate tube) and the term “outer” cladding or “overcladding” designates the cladding formed outside the deposition tube.
Deposition operations inside the deposition tube are typically chemical vapor depositions (CVD). A CVD deposition is performed by injecting mixtures of gas into a deposition tube and ionizing the mixtures. CVD-type depositions include modified chemical vapor deposition (MCVD), furnace chemical vapor deposition (FCVD), and plasma-enhanced chemical vapor deposition (PCVD). CVD techniques help to ensure that the OH— peak remains low and that attenuation at 1383 nanometers is therefore limited.
After layers corresponding to the core and the inner cladding have been deposited, the deposition tube (i.e., including the deposition layers) is converted into a solid rod by an operation referred to as “collapsing.” This produces the primary preform that is constituted by a solid rod (i.e., a solid rod including the collapsed deposition tube, inner cladding layers, and core layers). The primary preform is then overcladded, generally with grains of natural silica for reasons of cost. Overcladding may be performed by plasma deposition in which grains of doped or pure natural silica are deposited by gravity, melted, and vitrified on the periphery of the primary preform via a plasma torch.
Other techniques also exist for fabricating an optical-fiber preform. In this regard, the primary preform may be formed by outside deposition techniques, such as outside vapor deposition (OVD) or vapor axial deposition (VAD). During outside deposition techniques, no substrate tube is used. Rather, doped and/or undoped silica layers are deposited by directing precursor gases and a torch onto a starting rod.
As noted, various layers of a silica preform may be doped. During doping (i.e., component deposition), dopants are added to silica in order to change its refractive index. Germanium (Ge) or phosphorus (P) is used to increase the refractive index of silica. Germanium or phosphorus is often used for doping the central core of conventional optical fibers. Fluorine (F) or boron (B) is used to decrease the refractive index of silica. Fluorine is often used for forming depressed claddings.
Making a primary preform with a large, highly depressed cladding is difficult. For example, although a high temperature is required for making silica glass, it is difficult to incorporate fluorine in silica heated above a certain temperature.
PCVD techniques can be efficiently used to produce a depressed cladding inside a deposition tube. U.S. Pat. No. RE 30,635 and U.S. Pat. No. 4,314,833, each of which is hereby incorporated by reference in its entirety, describe PCVD techniques that allow fluorine to be significantly incorporated into silica in order to form highly depressed claddings. These patents describe that a deposition tube, made of pure silica or fluorine-doped silica, is mounted in a glasswork tower. The tube is then rotated while a gas mixture of silica and dopants is injected into the tube. The tube crosses a microwave cavity in which the gas mixture is heated locally. The microwave heating generates plasma by ionizing the gas mixture injected into the tube. The ionized dopants highly react with the silica particles, causing the deposition of doped silica layers inside the tube. The high reactivity of the dopants, generated by the microwave heating, enables a high concentration of dopants to be incorporated into the silica layers.
FIG. 1 illustrates a set refractive-index profile of a conventional PSCF. The depicted profile is a set profile that is representative of the optical fiber's theoretical profile. Constraints in the manufacture of the optical-fiber preform and the optical fiber, however, may result in a slightly different actual profile.
Those having ordinary skill in the art will recognize that the refractive indices of an optical fiber are equivalent to those of the optical-fiber preform from which the optical fiber is drawn. Furthermore, the radii of the core and cladding layers within an optical fiber are determined by the radii of the core and cladding layers within the optical-fiber preform from which the optical fiber is drawn. Thus, reference to an optical fiber's refractive-index profile can be readily extrapolated to the corresponding optical-fiber preform. That said, those having ordinary skill in the art will appreciate that the drawing process might cause an optical fiber's refractive index to deviate slightly from its corresponding optical-fiber preform.
The refractive-index profile of FIG. 1 depicts a central core having radius (a) and a refractive index, which corresponds to the refractive index of pure silica. The central core also has a refractive-index difference with the outer optical cladding (Dn1). An inner depressed cladding having an outer radius (rout) and a refractive-index difference with the outer optical cladding (Dninner) surrounds the central core. The inner cladding depicted in FIG. 1 is depressed, because it has a refractive index that is less than the refractive index of the outer optical cladding. The outer optical cladding is obtained by overcladding (e.g., by sleeving the primary preform). The outer cladding is generally formed of pure-silica glass and, therefore, has substantially the same refractive index as the central core in a PSCF. Typically, the outer cladding is formed from a substrate tube used to make the primary preform and/or from the overcladding used to reach the desired diameter ratio.
In the refractive-index profile depicted in FIG. 1, the fundamental mode LP01 is not completely guided and thus has additional losses, called leakage. To minimize these leakage losses, the percentage of energy propagating in the outer pure-silica cladding should be reduced. The ratio between the outer radius of the fluorine-doped inner cladding and the radius of the core (rout/a) should therefore be sufficiently high. In other words, the inner depressed cladding should be extended at least as far as a critical radius whose value is dependent on the core radius and the refractive-index difference between the core refractive index Dn1 and the refractive index of the inner cladding Dninner. For a standard SMF compliant with the ITU-T G.652 recommendations, it is thought that a ratio of eight or more between the outer radius of the inner depressed cladding and the radius of the core (i.e., rout/a>8) ensures good confinement of the optical signal in the central core and an acceptable level of leakage losses.
Commonly owned European Publication No. 2,312,350 and its counterpart U.S. Patent Publication No. 2011/0085770, each of which is hereby incorporated by reference in its entirety, disclose using a trench in the depressed cladding in order to limit the leakage losses of the LP01 mode. This design allows the substrate tube to be positioned closer to the central core. This solution, which is illustrated in FIG. 1 by the dotted lines, includes, within the depressed cladding, a trench having an inner radius (b), an outer radius (c), and a refractive index (Dn3). The depressed cladding has a first portion adjacent to the core having an outer radius (b) and a refractive index (Dn2) and a second portion adjacent to the substrate tube having an inner radius c and a refractive index (Dninner). This solution may be limited, however, because the substrate tube's inner radius (rout) cannot be easily reduced below 30 microns without adversely affecting the resulting optical fiber's propagation characteristics.
MCVD, FCVD, and PCVD techniques are satisfactory to obtain a good quality central core and a large, highly depressed inner cladding. These techniques, however, are costly whenever large capacity preforms are sought. The capacity of an optical-fiber preform is defined as the length of optical fiber that can be drawn from that preform. The greater the diameter of the preform, the greater its capacity. To reduce manufacturing costs, it is desirable to provide long lengths of optical fiber from one optical-fiber preform. It is therefore desirable to fabricate large-diameter preforms while complying with dimensional constraints relating to the diameter of the central core and the diameter of the optical cladding. After overcladding, the final preform (i.e., the optical-fiber preform) must present the same ratio of core diameter to cladding diameter as is to be achieved in the optical fiber drawn therefrom.
U.S. Patent Application Publication No. 2008/0031582 and U.S. Pat. No. 5,044,724, each of which is hereby incorporated by reference in its entirety, disclose using a fluorine-doped deposition tube to make the primary preform. This solution helps to minimize the quantity of fluorine-doped layers deposited inside the tube. International Publication No. 2010/003856 and its counterpart U.S. Patent Publication No. 2011/0100062, each of which is hereby incorporated by reference in its entirety, disclose the fabrication of fluorine-doped tubes by POD (Plasma Outside Deposition) or OVD.
When a fluorine-doped deposition tube is used, the depressed cladding of the primary preform is composed of the inner deposited cladding and the deposition tube itself. The ratio between the outer radius of the depressed cladding and the radius of the core can thereby be increased while limiting the quantity of deposition inside the tube. This solution, however, is not practical for very thick tubes because the deposition conditions change when a fluorine-doped tube is used instead of a pure-silica tube, ultimately limiting the reduction of the quantity deposited inside the tube.
U.S. Patent Application Publication No. 2007/0003198, which is hereby incorporated by reference in its entirety, discloses a hybrid process in which a rod used to form a germanium-doped core region is made by VAD or OVD and a cladding region is deposited inside a tube by MCVD. The core rod and the MCVD cladding tube are then assembled using a rod-in-tube technique. The optical fibers disclosed in this publication, however, do not have pure-silica cores or depressed claddings. As a result, these fibers do not face the same issues faced by PSCFs, namely achieving low attenuations at both 1383 nanometers and 1550 nanometers.
European Publication No. 2,003,476, which is hereby incorporated by reference in its entirety, discloses ring-assisted profiles in which the refractive index of the ring is close to that of the central core. The ring is designed to resonantly couple its modes with the higher-order modes of the central core. This solution prevents the central-core higher-order modes from propagating, ultimately improving the bend losses of the fundamental mode. These profiles, however, are not designed for pure-silica core fibers.